This invention relates to a drawing apparatus in the bedplate of a press comprising a pressure cheek which, during the forming of sheet metal parts, can be acted upon in a liftable manner by drawing cylinders against the drawing force of the press slide and for the ejecting of the sheet metal parts from the bottom part of the tool. This apparatus comprises pressure pins between the pressure cheek and a metal sheet holder of the bottom part of the tool for the transmission of drawing and ejector movements and forces.
In order to form deep-drawn pieces in a press, it is necessary to hold the metal sheet or the preform which was inserted into the tool and, by means of a yielding cushion, allow a counterforce ("drawing force") to act against the force caused during the downstroke of a slide of the press. During the drawing operation, the sheet metal part is held between a metal sheet holder in the bottom part of the tool and a counter contact in the top part of the tool.
In the German Patent Document DE 32 02 134 C1, a drawing apparatus for a press was described which comprises a pressure cheek that can be lifted and lowered in the bedplate of the press. The pressure cheek is situated below a holding plate for the bottom part of the tool and is supported in a pressure cylinder fixed to the bedplate of the press. By means of the pressure cheek, a number of pressure cylinders can be moved with respect to their height and act upon ejector pins in the bottom part of the tool by way of a pin plate. As a result of the force of the slide, the pressure cheek is lowered during the drawing operation by way of the pin plate and the ejector pins. After the drawing operation, the pistons of the pressure cylinders are acted upon, in the sense of an ejecting movement for the formed part with a slight pressure which is equal for all pressure cylinders, while the pressure cheek is still held in the lower position. The pressure cheek follows in a delayed manner in order to act upon the pin plate at the start of the next drawing operation.
In addition, a drawing apparatus in a press is known from the European Patent Document EP 0 173 755 A1 in which the metal sheet holder pressure is to be adapted in the different contact pressure ranges for the deep-drawn part. For this purpose, a number of pressure cylinders are used which are fixedly installed in the bedplate of the press and the piston rods of which can be moved with respect to their height by way of a pressure cheek which acts jointly on all piston rods. The piston rods which act on the metal sheet holder at the other end, during the slide movement, can be lowered below the pressure which can be variably controlled via valves in the pressure chambers.
In the German Patent Document 37 17 768 A1, a drawing apparatus in the bedplate of a press was described in which the drawing and the ejecting of the sheet metal parts is to take place either by means of metal sheet holders or by means of a drawing die. A change of the operating method is required for this purpose. In the drawing apparatus, a pressure cheek is acted upon by drawing cylinders by way of a liftable and lowerable carriage. For this purpose, pressure rods are fixedly disposed in the carriage in the extension of the piston rods of the drawing cylinders. The pressure rods are moved out by the pressure cheek in order to rest against the metal sheet holder. Mechanical couplings for the selective uncoupling and coupling of the drawing apparatus and the ejector apparatus from and to the pressure cheek are arranged between the pressure cheek and each of the pressure rods and between the pressure cheek and the lengthened piston rod of the ejector apparatus.
Similar to these previously known drawing apparatuses are those which have a number of pressure pins that only become active together or act upon a metal sheet holder while they are acted upon by drawing cylinders. The number of drawing cylinders is lower than the number of pressure pins, however, without the possibility of carrying out a selection of pressure pins to be acted upon.
An object of the invention is to provide an apparatus which enables selection of the pressure pins acting upon the metal sheet holder in order to hereby fulfill the requirements concerning the size of a sheet metal part, its shape and the free forms and to obtain, for example, also mirror-symmetrical sheet metal parts. Another object of the invention is to provide apparatus which, by the selection of specific pressure pins and correspondingly fewer controlled drawing cylinders, carries out the ejector operation with less energy, while the drawing cylinders which are not controlled during the ejector operation are controlled in a later phase.
This object is achieved by the present invention which provides a drawing apparatus in a bedplate of a press that has top and bottom tool parts and a press slide, the bottom tool part having a metal sheet holder. The drawing apparatus comprises drawing cylinders and a pressure cheek which, during the forming of sheet metal parts, can be acted upon in a liftable manner by the drawing cylinders against a drawing force of the press slide and for ejecting the sheet metal parts from the bottom tool part. Pressure pins are arranged between the pressure cheek and the metal sheet holder of the bottom tool part for transmitting drawing and ejector movements and forces. Pressure cylinders are disposed in the pressure cheek, the number of pressure cylinders being equal to the number of pressure pins, each pressure cylinder having a pressure cylinder piston rod. The pressure cylinders are arranged in a vertical course with the same axis with respect to a corresponding one of the pressure pins and has piston surfaces which are acted upon from the bottom by pressure so as to cause each of the pressure cylinder piston rods to contact the corresponding pressure pin from below.
An advantage of the present invention is that the metal sheet holding operation during the drawing as well as the ejector operation is influenced by the fact that the pressure cylinders in the pressure cheek are selectively controlled, whether acted upon or not acted upon. The drawing may take place with a lower number of drawing cylinders while the individual drawing force in each case is higher and the selective control of pressure pins is utilized. At the same time, the individual drawing force and therefore the dimensioning of each of the drawing cylinders is determined to be lower.
In certain embodiments of the invention, not only a reduction of the overall pipe cross-section to the valves is achieved, but also elasticities of the circuit structure are significantly reduced. This is due to the reduction of the distances between the respective pressure space of a pressure cylinder and the pertaining on-off valve.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.